1. Field of the Invention
This invention relates generally to high-power microwave generators and more particularly to high-power microwave systems employing a phase-locked array of inexpensive commercial magnetrons.
2. Description of the Related Art
The conventional magnetron (herein also denominated “magnetron tube”) is a well-known and very efficient device used to convert stored electrical energy into microwave-frequency alternating currents. Magnetron operating principles have been known since at least 1921 and magnetrons have been used in extensively in microwave radars since the first pulsed resonant cavity magnetron (3 GHz) was developed by the British in 1940. Today, inexpensive mass-produced magnetrons (herein also denominated “commercial magnetrons”) can be found in every home possessing a microwave oven.
A typical single-body magnetron tube known in the art is a coaxial vacuum device consisting essentially of an external cylindrical anode (which attracts electrons) and an internal, coaxial cylindrical cathode (which emits electrons). In a typical design, the anode is grooved to form resonator cavities disposed to form a symmetric series of vanes. In operation, an electric potential (“anode voltage”) is placed across the evacuated annulus formed between the anode and cathode. Simultaneously, a constant axial magnetic field is created in the evacuated annulus that serves to cause electrons emitted by the cathode responsive to the anode voltage to travel around the cathode in paths that are ‘influenced’ by RF fields in the anode resonant cavities. With appropriate conditions, electrons following these paths form rotating ‘spokes’ of space charge that interact with the anode resonator fields in such a way as to induce displacement currents in each resonator cavity. As a spoke of electrons approaches an anode vane, it induces a positive charge in that vane. As the electron spoke passes, the positive charge diminishes in the first vane while another positive charge is being induced in the next vane. The physical structure of the anode forms the equivalent of a series of high-Q resonant inductive-capacitive (LC) circuits. The vanes are alternately strapped together to effectively connect the LC circuits in parallel. The induced displacement currents of each resonator are coupled to a tuned output cavity by any of various means, and from there are coupled to a primary output waveguide that conducts the microwave energy into an energy absorbing or transmitting load. The number and shape of the resonator cavities and the dimensions of the anode and cathode are most often selected by the designer based on scaled values from previous magnetron designs that are, in turn, selected for their appropriateness for the given application. Design features that might cause one type of resonator configuration to be preferable over another type include operating characteristics such as the “pushing factor,” which denominates a measure of the output frequency variation arising from anode voltage fluctuations, and the “pulling factor,” which denominates a measure of the output frequency variation arising from changes in RF load impedance. Clearly, as is well-known in the art, the magnetron tube is a complex resonant ‘system’ for energy conversion whose precise operating parameters, for example frequency and efficiency, depends on many different design and load factors and may accordingly be somewhat intractable.
It is also well-known in the art that the output energy from a magnetron can be ‘locked’ in frequency and phase to that of an externally-applied signal that is properly ‘injected’ into the magnetron's resonant structure with an appropriate amplitude within a limited ‘locking range’ of frequencies. The basic equation that describes this injection-lock behavior for small injection magnitudes was derived by Adler (R. Adler, “A Study of Locking Phenomena in Oscillators,” Proc. IRE, Vol. 34, pp. 351–357, June 1946):
                                          Δ            ⁢                                                  ⁢                          ω              L                                            (                                                            ω                  o                                /                2                            ⁢              Q                        )                          =                                            V              L                                      V              o                                ⁢          sin          ⁢                                          ⁢          α                                    [                  Eqn          .                                          ⁢          1                ]            where ΔωL is half of the maximum locking range, ωo is the ‘natural’ frequency of the oscillator, Q is the quality factor of resonant circuit of the oscillator, VL is the injection input level, Vo is the oscillator output level, and a is the steady-state phase difference between the injected signal and the output signal.
A single magnetron tube operating continuously is presently subject to practical output power limits of about 1 MW, which can be attained only with a very large and expensive device supported by large and expensive external cooling systems. In principle, Eqn. 1 above teaches that larger microwave energy outputs suitable for applications such as radar, power transmission and directed energy weapons, can be obtained by means of the coherent combination of synchronous output energies from a plurality of magnetron tubes. Moreover, in principle, for any particular system output power, the system cost, size, and reliability can be improved significantly by phase-locking many smaller magnetrons. In view of these conceptual advantages, early practitioners in the art attempted to achieve higher system output power by phase-locking a plurality of separate magnetron tubes. For example, several early efforts were made to achieve injection phase-locking of several distinct or separate magnetron tubes with a common master input signal with varying levels of success. Another early effort was made to achieve bootstrap phase-locking of several distinct magnetron tubes arranged in a hexagonal array with pair-wise waveguide connections between them, by energizing them simultaneously without benefit of a common master input signal. The phase-locking effect of such pair-wise waveguide communication between hexagonally-arrayed magnetron tubes was found to be achievable only at the expense of dedicating an evacuated port between each adjacent pair of individual magnetrons. Neither approach is presently considered in the art as a useful solution to the magnetron tube phase-locking problem.
FIG. 1 illustrates an exemplary solution to the dual magnetron phase-locking problem from the prior art, showing a dual phase-locked magnetron system 10, including a separate drive signal source 12, the two magnetron tubes 14–16 each having a respective output 18–20, and the respective three-port power circulators 22–24 each disposed to couple output power from the respective magnetron tube output 18–20 to the respective primary coupling path 26–28 from which the resulting microwave energy is coupled to free space (not shown). Power from drive signal source 12 is introduced at the port 30 and respective portions of the respective magnetron tube output powers are introduced at the ports 32–34 for distribution to circulators 22–24, from which the respective energies are injected into the respective magnetron tube outputs 18–20 to “pull” the respective magnetron tubes 14–16 into phase-lock with drive signal source 12. Circulators 22–24 and drive signal source 12 represent a substantial weight, volume, cost, and complexity burden for dual phase-locked magnetron system 10; so substantial that a single magnetron tube having twice the individual power rating of magnetron tubes 14–16 may represent a more cost-effective embodiment for system 10. Moreover, the power losses in power circulators 22–24 are also a significant burden in some applications.
FIG. 2 illustrates a well-known injection magnetron phase-locking method from the prior art. A three-port high-power circulator 36 is coupled to the output cavity (not shown) of the magnetron tube 38 by way of the primary coupling path 40. An injection signal generator 42 produces an injection signal fi and couples it to the second circulator port 44, where it is injected into primary coupling path 40 to “pull” the magnetron tube output signal fo into phase-lock with injection signal fi. Output signal fo at circulator 36 is coupled to the load 46 by the third circulator port 48. Circulator 36 must be fabricated to handle the primary output power from magnetron tube 38 and is generally disadvantageously inefficient. For effective phase-locking, the requisite level of injection signal fi depends on the spectral bandwidth (also known as the quality factor ‘Q’) of magnetron tube 38 as defined by Adler in the reference cited above in connection with Eqn. 1. This technique is expensive but it permits the precise control of the somewhat unruly magnetron tube output signal fo by injection signal fi from injection signal generator 42, which may be as precise and stable as desired.
FIG. 3 illustrates a well-known phase-locked loop (PLL) magnetron phase-locking method from the prior art. The magnetron tube output signal fo from the output cavity (not shown) of the magnetron tube 50 is coupled by the primary coupling path 52 through a loop coupler 54 to the load 56. A reference signal generator 58 provides a reference signal fr to a phase detector 60, which also accepts a sample of magnetron tube output signal fo from loop detector 54. Phase detector 60 compares reference signal fr and magnetron tube output signal fo to produce an error signal fe, which is coupled to the anode current control element 62 for magnetron tube 50. Loop coupler 54, anode current control element 62 and phase detector 60 operate as a PLL circuit in the well-known manner. With the proper loop stability, adjusting the anode current to magnetron tube 50 responsive to error signal fe soon brings magnetron tube output signal fo into phase-lock with reference signal fr. This technique is also expensive but it permits the precise control of the somewhat unruly magnetron tube output signal fo by reference signal fr from reference signal generator 58, which may be as precise and stable as desired and this method may avoid some of the power losses noted in circulator 36 of FIG. 2.
Other practitioners in the art have more recently proposed solutions to the magnetron array phase-locking problem. For example, in U.S. Pat. No. 4,571,552, Brown discloses a technique for phase locking a magnetron output signal with a frequency source signal that is obtained by comparing the output signal phase to a source signal phase to obtain an error signal that is applied to a winding of the magnetron magnet to thereby change the flux applied to the magnetron tube, while the magnetron output signal frequency is also “pulled” by the source signal injected into the magnetron tube by way of a three-port circulator. Brown's technique requires an additional magnet winding, external error detection circuitry, and an external three-port high-power circulator; all additional to the magnetron tubes themselves.
In U.S. Pat. No. 4,634,992, Brown discloses an alternative technique for combining the high output power of two magnetron tube amplifiers using a novel (“Magic T”) microwave circuit to reduce the power dissipated by a low-power ferrite circulator connected between the input signal source and the Magic T circuit. Brown is obliged to add external phase and amplitude comparators to control the magnetron tube outputs so that they may be coherently combined in the Magic T circuit. Brown's alternative technique also requires significant additional components, including an additional Magic T circuit, external phase and amplitude error detection and correction circuitry and an external three-port low-power circulator; all additional to the magnetron tubes themselves.
With a different approach, in U.S. Pat. No. 5,162,698, Kato et al. disclose a cascaded magnetron device having a series of tubular anode elements placed end to end in a linear cascade extending along at least part of an elongate cathode shank. Each adjacent pair of anode elements is separated by a conductive, annular pin-down disc, and the cathode shank has a series of spaced bands of field-emitting material separated by non-emitting regions, each band being located within a respective one of the anode elements and spaced inwardly from the ends of that element. Suitable power inputs and magnetic field generators are provided for producing electron emission and oscillation in the interaction zone between each emitting band and the respective anode element surrounding that band, and suitable extraction devices are provided for extracting power from each of the interaction zones, thereby phase-locking the cascaded magnetron bodies. In effect, Kato et al. propose a single device having a plurality of magnetron tube cavities disposed in a fixed coaxial relationship, which neither resolves nor even considers the problem of coupling a plurality of separate commercial magnetron tubes for effective high power operation.
These magnetron tube phase-locking efforts were motivated primarily by the high power requirements of, e.g., radar transmitters, particle accelerators and space-power-generators, where precise phase, power, and frequency control is imperative. For example, electron accelerators require microwave power supplies having phase stability within 0.1 to 0.2 degrees of nominal. A secondary motivation for these efforts is the universal availability of inexpensive commercial magnetron tubes. Finally, magnetron tubes are generally preferred because even a relatively expensive high-power magnetron tube can be manufactured for less than half the cost per kilowatt of, for example, a klystron. Available solutions to the magnetron phase-locking problem, such as the external phase-control circuitry described in the two Brown patents (U.S. Pat. Nos. 4,634,992 and 4,634,992), all introduce unwelcome burdens of complexity and cost into any microwave system employing a plurality of separate magnetron tubes. Although such solutions may succeed in stabilizing frequency, phase, and power output from a plurality of phase-locked magnetron tubes, the attendant burdens are disadvantageous for many microwave power applications (such as directed energy weapons) that may be more tolerant of small variations in output phase, power and frequency.
There is accordingly a clearly-felt need in the art for an efficient and inexpensive means for phase-locking a plurality of simple and inexpensive commercial magnetron tubes to permit the coherent combining of the resulting plurality of energy outputs. These unresolved problems and deficiencies are clearly felt in the art and are solved by this invention in the manner described below.